The invention relates to apparatus and a method for periodically sampling a liquid and particularly to sampling wastewaters.
The purpose of sampling a liquid is to obtain a representative portion of the liquid from which its average composition can be determined. When sampling a flowing liquid, its average composition over a specific period of time is required (for example, a 24 hour average). This is normally done by compositing a set of individual sub-samples taken with sufficient frequency to include all the effects of changes in composition of the flow during the period of the sample. The more frequent the taking of these individual samples, the more representative will be the complete sample.
The frequency of the sampling may be at regular time intervals or at time intervals in inverse proportion to the rate of flow of the sampled liquid. The latter case being flow-proportional sampling.
The individual sub-samples may be composited in a single container to give a gross average or alternatively, may be distributed into an array of separate containers to aportion the flow from hour to hour.
When sampling wastewaters it is important to include any solids waste that is being carried by the water. Wastewaters normally flow along open channels or ventilated drains and samples of this liquid are usually obtained by pumping a small volume into one or more containers at predetermined time intervals. Wastewaters often contain settleable particulate matter like washings and grit and also contain fine solids matter in suspension within the liquid. To obtain a representative sample of these solids, the wastewater must be pumped at high velocity, to prevent the settleable matter from being left behind, and be transported through tubing of sufficient bore to avoid the risk of internal blockage within the sampling apparatus.
Wastewaters may also contain solids matter in the form of debris like rags, leaves and other coarse material which is not normally required to be included in the sample. These solids must not be allowed to obstruct the inlet or the internal parts of the sampling apparatus.
Sampling apparatus for wastewaters have hitherto been based on powerful pumps delivering large individual samples through large bore delivery tubes. However, if the total sample volume is much over one gallon, the sample becomes difficult to carry and therefore most practical sampling devices have a limited capacity of about one gallon. So if the individual samples are of large volume, then the frequency of these samples must be correspondingly low and so the accuracy of the total sample may also be low.